1. Field of the Invention
The present invention relates to a method and a system for controlling the output of a fuel cell power generator having a fuel cell which generates a direct current or DC output for conversion by an inverter into an alternating current AC output to be fed to a power system. More particularly, the invention relates to an output controlling method and system for protecting the fuel cell from overload.
2. Description of the Prior Art
Recently, in order to fully introducing a fuel cell power generator system as a new type of energy-distributed power supply facilities, it is desired to supply power stably, and to run the generator system at a high operating efficiency. For this purpose, in the case of the fuel cell power generator system cooperatively linked with a power system, a power supply is controlled in such a way that a DC output from a fuel cell is converted into an AC output under a constant output control by means of a reverse converter (e.g., an inverter). The AC output thus converted is fed into the power system.
FIG. 4 is a schematic block diagram illustrating a conventional output controlling system for controlling the output of a fuel cell power generator. In FIG. 4, a fuel reformer 1 reforms raw fuels (e.g., natural gas) with water vapor to generate a fuel gas 1F. The fuel gas 1F thus reformed by the fuel reformer 1 is fed to a fuel cell 2. The fuel cell 2 receives the fuel gas 1F and reactant air to generate a DC output I.sub.FC. An inverter 3 or reverse converter converts the DC output I.sub.FC into an AC output I.sub.out. The power output P.sub.out corresponding to the AC output I.sub.out is fed to a power system (not shown). The output controlling system comprises: an ammeter 9 for detecting the output AC current value I.sub.out of the inverter 3, a wattmeter 8 for detecting the output power P.sub.out corresponding to the output current I.sub.out ; an output control regulator 5 for controlling the detected value P.sub.out of the output power of the wattmeter 8 as close to the output set value P.sub.s of an output setting unit 4 as possible; a current command computing unit 6 for updating a current command value on the basis of the output signal from the output control regulator 5; and a controller 7 for the inverter 3 for controlling the output AC current value I.sub.out of the inverter 3 as close to the current command value I.sub.sc as possible. The controller 7 typically includes an ACR minor loop for delay compensation and a PWM control circuit, whereby the inverter 3 is subjected to constant output control with reference to the output set value P.sub.s.
FIG. 5 is a graphic representation of typical output voltage vs. output current characteristic curves associated with a fuel cell. As indicated by a curve 101 in FIG. 5, the fuel cell are known to exhibit a drooping output voltage vs. output current characteristic even when supplied with sufficient quantities of fuel gas and reactant air. That is, the output voltage tends to drop as the output current is being increased. In FIG. 4, in case of a sudden increase in the output set value P.sub.s, the inverter 3 responds immediately to request the fuel cell 2 to raise the output power P.sub.out. However, due to a delay in the increase of the fuel gas 1F supplied from the fuel reformer 1 involving chemical reaction and substance movement, the fuel cell 2 is likely to generate power in a transient condition of fuel gas shortage. Hence the pronounced drooping output voltage vs. output current characteristic represented by a curve 102 is shown in FIG. 5. A similar drooping characteristic also becomes apparent if the fuel reformer 1 has failed, leading to a temporary decrease in the supply of the fuel gas 1F.
If an efficiency of the inverter is neglected, the output power P.sub.out of the fuel cell power generator is generally expressed as cell voltage multiplied by cell current. For example, suppose that in FIG. 5, a fuel gas shortage takes place at point A on the curve 101 (P.sub.out =V.sub.A .times.I.sub.A) where the fuel cell is normally operating, the conventional fuel cell power generator whose inverter is under constant output control as depicted in FIG. 4 attempts to increase the current to I.sub.B to compensate for the drop of the voltage V.sub.A down to V.sub.B, thereby keeping the output power P.sub.out constant at point B on the curve 102. This results in a performance curve 103 connecting points A to B, respectively, on the two preceding curves, the curve 103 reflecting the fuel gas shortage. Since the greater the fuel gas shortage, the higher the density of the current generated by the fuel cell, operating the fuel cell in the state above can deteriorate the performance characteristic of the electrode catalyst by agglomerating its particles or the like. As a result, shortened service life of the fuel cell is shortened.